Apr 5, 2017

This happened to me recently and I suspect the recent re-introduction of Google Maps real-time location sharing to be related: my phone would no longer transfer the phone calls to the car hand-free unit.

It turns out that the "Call Audio" option for that Bluetooth device became disabled and could not be re-enabled by any mean.

The culprit is an option called "Bluetooth scanning", which, since Nougat, has been relocated in the upper right menu under Settings > Location

Once there, just disable the "Bluetooth scanning" option.

Now, go back to the Bluetooth settings and enable "Call audio".
This time the option will remain enabled.